Embodiments of the invention relate generally to Fast Spin Echo (FSE) magnetic resonance (MR) imaging and, more particularly, to a system and method of split-echo, split-blade data collection for PROPELLER MR imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net “transverse magnetization” Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
FSE imaging is an imaging technique commonly used as an efficient method of collecting MR data with minimal artifact. A FSE pulse sequence uses multiple refocusing RF pulses to generate an echo train after a single excitation RF pulse. Generally, FSE requires that the phase of all the refocusing RF pulses be substantially identical to that of the initial transverse magnetization after excitation, commonly referred to as the Carr-Purcell-Meiboom-Gill (CPMG) condition. If this condition is not met, the resulting MR signal is generally affected by destructive echo interference, resulting in unstable echo train and signal cancellation. Specifically, the signals of FSE echo train are from the contributions of a large number of so-called pathways. Under CPMG condition, the signals from different pathways reinforce one another thus the signal intensities of the later echoes are stably maintained. If the CPMG condition is violated, these signals from different pathways no longer line up and destructive interference occurs. Accordingly, the resulting signals will decay rapidly in successive echoes.
As a result, FSE imaging with diffusion weighted imaging (DWI) may be difficult, in general, since even minute patient motion or system vibration during application of diffusion weighting gradients leaves the spins with a spatially varying and unknown starting phase prior to refocusing RF pulse train. A number of imaging techniques have been developed that modulates the phase of the refocusing pulses to attempt to delay the echo signal decay. However, these known techniques have been shown to prolong the signal magnitude, but, in general, cause a spatially varying phase which alternates between even and odd echoes, which makes combining the two sets of echoes difficult.
FSE imaging has been implemented with PROPELLER (Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction) technique to address the phase discrepancy among echo trains. PROPELLER encodes MR signals by collecting data during an echo train such that a rectangular strip, or “blade”, through the center of k-space is measured. This strip is incrementally rotated in k-space about the origin in subsequent echo trains, thereby the phase inconsistency among blades can be corrected by using the overlapped data at central k-space. PROPELLER can mitigate the violation of CPMG condition by RF phase modulation combined with split-blade acquisition. However, for this type of techniques, the flip angle of refocusing RF pulses is preferred to be high enough to stabilize the echo train, which results in long scanning time at high field strength due to the limit of specific absorption rate (SAR). Also, the number of the acquired lines of each blade is just half of the echo train length, which is not desirable for PROPELLER reconstruction that requires wide blade width to avoid potential artifacts.
SPLICE PROPELLER is another imaging approach that can mitigate non-CPMG artifacts. SPLICE uses an imbalanced readout gradient and an extended acquisition window to split each echo in an echo train into an echo pair. Two k-space datasets have to be acquired simultaneously, and two intermediate PROPELLER images have to be reconstructed by separately using the first and second echoes in each echo pair. As such, SPLICE PROPELLER has the disadvantage of decreased data acquisition efficiency, due to the requirement of simultaneous acquisition of two k-space datasets.
It would therefore be desirable to have a system and method capable of acquiring split-echo split-blade data while mitigating non-CPMG artifacts. Specifically, it would be desirable to provide a FSE method that generate echo pairs to create two blades for each echo train, without the reduction of blade width to avoid the potential narrow-blade artifacts. Also, it would be desirable to place the two split blades into a single k-space such that the data acquisition efficiency is not sacrificed.